The invention relates to a method for preparing a mechanically stable zeolite catalyst composition possessing a refractory oxide binder of low acidity, e.g., a silica binder.
The term "zeolite" as used herein designates the class of porotectosilicates, i.e., porous crystalline silicates, which contain silicon and oxygen atoms as the major components. Other framework components can be present in minor amount, usually less than about 14 mole %, and preferably less than 4%. These components include aluminum, gallium, iron, boron, etc., and combinations thereof. The crystalline aluminosilicates constitute an especially well known type of zeolite.
It is well known that extrusion is one way of obtaining a zeolite-containing material which has a high degree of strength for various applications, both catalytic and noncatalytic. Some aluminosilicate zeolites have long been used as catalysts for a wide variety of organic conversion processes. In general, aluminosilicate zeolites are incorporated with a matrix, or binder, material in order to impart mechanical stability hereto. The most commonly used matrix materials have included alumina and/or clays since these materials are fairly easy to extrude and provide extrudates of good physical strength.
It has long been recognized that silica is a desirable matrix and that it possesses advantages over alumina for some catalytic reactions. In this connection, U.S. Pat. No. 4,013,732 specifically discloses ZSM-5 with a silica matrix and U.S. Pat. Nos. 3,843,741 and 3,702,886 broadly disclose the use of ZSM-5 with a silica matrix.
U.S. Pat. No. 4,582,815 describes a method for preparing silica-rich solids said to possess improved crush strength compared to that of known silica-bound materials. The method comprises mixing silica-rich solids, preferably a mixture of silica with a zeolite such as ZSM-4 (Omega), ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-48, Beta, X, Y, L, ferrierite, mordenite, dachiardite, clinoptilolite, offretite, erionite, gmelinite, chabazite, etc., with water and an alkali metal base such as sodium hydroxide or a basic salt such as an alkali-metal carbonate, borate, phosphate, silicate, etc., as an extrusion aid followed by mulling, extruding and subsequently drying the extrudate. It is thought that substitution of alkali metal for hydrogen in the silanol groups on the surfaces of siliceous materials such as the foregoing zeolites is responsible for their improved crush strength. The resulting extrudate is said to possess superior crush strength and sufficient integrity to withstand treatments with acids so that is is possible to steam, acid extract or calcine them. To avoid trapping the alkali metal of the extrusion aid in the extrudate, the alkali metal is ordinarily removed by exchange under acidic conditions using dilute nitric acid in 1M ammonium nitrate solution.
Silica-bound zeolite catalysts prepared in accordance with the method described in U.S. Pat. No. 4,582,815 are indicated to be useful in hydrocarbon conversions such as hydrocracking, isomerization, hydrogenation, dehydrogenation, polymerization, reforming, catalytic cracking and catalytic hydrocracking.